Substance Use Disorders (SUD) involve changes in dopamine (DA) and glutamate (GLU) transmission which imply non-adaptive plastic changes in corticostriatal circuitry. Our previous studies have demonstrated a key role of adenosine (ADE) in the modulation of striatal DA and GLU transmission, largely mediated by functional complexes of different adenosine and dopamine receptor subtypes, G protein-coupled receptor (GPCR) heteromers. At the postsynaptic level, ADE A1 and A2A receptors (A1R and A2AR) are co-segregated with DA D1 and D2 receptors (D1R and D2R) in the GABAergic striatonigral and striatopallidal neurons, respectively, forming A1R-D1R and A2AR-D2R heteromers. ADE and DA receptors are also localized at the presynaptic level, in the corticostriatal terminals, but forming GPCR heteromers of either ADE or DA receptor subtypes, mainly A1R-A2AR and D2R-D4R heteromers, which exert a significant control of GLU release. Our research project deals with the study of the functional and pharmacological properties of these striatal GPCR heteromers, their role in SUD and other neuropsychiatric disorders that involve dysfunctional DA and GLU transmission (Parkinsons disease, L-dopa-induced dyskinesia, Huntington disease HD, schizophrenia, attention deficit hyperactivity disorder ADHD and Restless Legs Syndrome RLS), and their possible role as targets for the treatment of these neuropsychiatric disorders. An additional assumption is that their different pathogenesis and symptomatology depend on different changes in the properties or stoichiometry of ADE and DA receptor heteromers (density and percentage of receptor forming and not forming heteromers). For instance, we have previously reported evidence that indicate that D4.7R, the product of a human polymorphic variant of the D4R gene (DRD4) repeatedly associated with ADHD, forms less functional heteromers with D2R. When performing immunohistochemical and optogenetic-microdialysis experiments in knock-in mice expressing a D4R with the long intracellular domain D4.7 and comparing with the wild-type mouse D4R receptor, the expanded intracellular domain of the humanized D4 receptor conferred a gain of function, blunting optogenetic and methamphetamine-induced corticostriatal glutamate release (1). The results demonstrate a key role of the D4R in the modulation of corticostriatal glutamatergic neurotransmission, which depends on its degree of heteromerization with D2R. Furthermore, the results also imply that enhanced D4R-mediated control of corticostriatal GLU transmission constitutes a vulnerability factor of ADHD and most probably other neuropsychiatric disorders. Deficits of sensorimotor integration with periodic limb movements during sleep (PLMS) and hyperarousal with sleep disturbances in RLS constitute two pathophysiologically distinct but interrelated clinical phenomena (2). Brain iron deficiency (BID) is considered as a main pathogenetic mechanism in RLS. Rodents with BID (which can only be obtained with a severe iron-deficient diet starting at the postweaning period) represent a valuable pathophysiological model of RLS, although they do not display motor disturbances. Nevertheless, they develop the main neurochemical changes found in RLS, such as decrease in striatal D2R density. On the other hand, rodents with BID exhibit the characteristic pattern of hyperarousal in RLS, providing a tool to find the link between BID and sleep disturbances in RLS. In our most recent study with this animal model we provided evidence for dysfunction of ADE transmission, which is in fact a key endogenous sleep-promoting factor (3). Thus, we demonstrated that BID in rodents produces a significant downregulation of A1R in the cortex, a very plausible mechanism of hyperarousal in RLS (3). Furthermore, ADE could constitute the link between hyperarousal and PLMS in RLS, since functional downregulation of A1R was also found in the striatum (3). Together with our previously demonstrated upregulation of both pre- and postsynaptic striatal A2AR in the same animal model, this would imply a dramatic change in the stoichiometry of A1R-A2AR heteromers versus A2AR homomers and a facilitation of corticostriatal GLU transmission. We have therefore postulated that A1R downregulation induced by BID is a key pathogenetic factor involved in both PLMS and hyperarousal in RLS (2,3). We have also postulated that not only striatal presynaptic A1R-A2AR heteromers, but also D2R-D4R heteromers, localized in striatal GLU terminals are important potential targets for the treatment of PLMS in RLS (4). Previous studies have found a preferential decrease in the striatal A2AR and D2R versus D1R density in the early stages of HD. The decrease in the density of A2AR seems to involve a downregulation independent of neurodegeneration, related to alterations in the normal regulation of transcription of the A2AR gene. A2AR is also usually shown to be downregulated in early stages of neuropathological development in rodent models of HD (5) and several preclinical and clinical studies have suggested that the decrease in A2AR density or its tonic activation could be involved in the pathogenesis of HD. Genetic and epidemiological studies also support the involvement of A2AR. Several polymorphisms in the A2AR gene (ADORA2A) and also caffeine (non-selective adenosine receptor antagonist) intake have been significantly associated with a reduced age of onset of HD. We have now found evidence of a significantly reduced tone of striatal adenosine in two rodent models of HD, a decrease in extracellular concentration of adenosine associated with upregulation of the equilibrative nucleoside transporter ENT1 (5). Importantly, the ENT1 transcript was also found to be significantly upregulated in HD disease patients at an early neuropathological severity stage and was differentially coexpressed (gained correlations) with several other genes in HD disease subjects compared to the control group (5). Our study demonstrates that ENT1 and adenosine constitute biomarkers of the initial stages of neurodegeneration in HD disease and also predicts that ENT1 blockers could provide new therapeutic agents to delay the progression of the disease (5,6). In addition to striatal GPCR heteromers, we are exploring the possibility of GPCR heteromers localized in the ventral tegmental area (VTA) that modulate DA cell function and, therefore, could also constitute targets for SUD and for the treatment of loss of control of food intake. So far, we have focused our studies to GPCR of hormones and neuropeptides (7). We previously described heteromers of orexin and corticotropin-releasing factor receptors and we have now been able to demonstrate that the main population of mu-opioid receptors that exert an inhibitory modulation of DA cell function in the VTA (somatodendritic DA release and MAPK activation) form heteromers with galanin Gal1R (8). Within this GPCR heteromer, galanin allosterically inhibits agonist-mediated mu-opioid receptor signaling (8). This heteromer represents a potential new target for opioid use disorder.